The present invention relates to a processing solution for forming a hexavalent chromium free and corrosion resistant conversion film on zinc or zinc alloy plating layers, a hexavalent chromium free and corrosion resistant conversion film and a method for forming the hexavalent chromium free and corrosion resistant conversion film.
As methods for rust preventing the surface of a metal, there has been known a zinc or zinc alloy-plating method. However, it is not possible to ensure sufficient corrosion resistance of the metal by such plating alone. For this reason, there has widely been adopted, in this industrial field, the treatment with chromic acid containing hexavalent chromium or the so-called chromate treatment after the plating. Nevertheless, it has recently been pointed out that the hexavalent chromium may adversely affect the human body and the environment and there has correspondingly been such a strong and active trend that the use of hexavalent chromium should be controlled.
As one of the substituent techniques therefor, the formation of a corrosion resistant conversion film, in which trivalent chromium is used, has been known. For instance, Japanese Examined Patent Publication (hereunder referred to as xe2x80x9cJ.P. KOKOKUxe2x80x9d) No. Sho 63-015991 discloses a method, which comprises the step of treating the surface of a metal with a bath containing a mixture of trivalent chromium and a fluoride, an organic acid, an inorganic acid and/or a metal salt such as cobalt sulfate. However, a fluoride is used in this plating bath and therefore, a problem of environmental pollution would arise. In addition, J.P. KOKOKU No. Hei 03-010714 discloses a method, which makes use of a plating bath comprising a mixture of trivalent chromium and an oxidizing agent, an organic acid, an inorganic acid and/or a metal salt such as a cerium salt. However, this method makes use of an oxidizing agent and cerium and therefore, the trivalent chromium may possibly be oxidized into hexavalent chromium, during the processing and/or the storage of the bath.
Furthermore, Japanese Un-Examined Patent Publication (hereunder referred to as xe2x80x9cJ.P. KOKAIxe2x80x9d) No. 2000-509434 discloses a method, which comprises the step of treating the surface of a metal using a plating bath comprising 5 to 100 g/L of trivalent chromium and nitrate residues, an organic acid and/or a metal salt such as a cobalt salt. This method uses, for instance, trivalent chromium in a high concentration and the plating operation is carried out at a high temperature. Therefore, this method is advantageous in that it can form a thick film and ensure good corrosion resistance. However, the method suffers from a problem in that it is difficult to stably form a dense film and that the method cannot ensure the stable corrosion resistance of the resulting film. Moreover, the processing bath contains trivalent chromium in a high concentration and also contains a large amount of an organic acid. This makes the post-treatment of the waste water difficult and results in the formation of a vast quantity of sludge after the processing. Although one can recognize that it is advantageous to use a processing solution free of any hexavalent chromium for ensuring the environmental protection, the method suffers from a serious problem in that it may give a new burden to the environment such that the method generates a vast quantity of waste.
Moreover, there have been proposed a method for processing the surface of a metal with a bath containing trivalent chromium in a low concentration and an organic acid and a metal salt such as a nickel salt (U.S. Pat. No. 4,578,122) and a processing method, which makes use of a bath containing trivalent chromium in a low concentration and an organic acid (U.S. Pat. No. 5,368,655). However, these methods never ensure sufficient corrosion resistance of the resulting film as compared with the conventional hexavalent chromate treatment.
As has been discussed above in detail, it has been known that if zinc or a zinc alloy are immersed in a solution of a trivalent chromium salt, a chromium-containing film is formed thereon.
However, the resulting film is insufficient in the corrosion resistance effect. Therefore, it is necessary to increase the thickness of the resulting film by increasing the chromium concentration in the processing solution, raising the processing temperature and extending the processing time in order to obtain a film having the corrosion resistance effect identical to that achieved by the conventional corrosion resistant conversion film derived from hexavalent chromium. However, this leads to an increase in the energy consumption and in the quantity of the waste sludge, which is not desirable from the viewpoint of the environmental protection.
Accordingly, it is an object of the present invention to provide a thin, hexavalent chromium free film, which is applied onto the surface of zinc or zinc alloy plating layers, which have corrosion resistance identical to or higher than that achieved by the conventional hexavalent chromium-containing conversion film and which can be formed using a processing solution having a low concentration. More specifically, it is an object of the present invention to provide a hexavalent chromium free, corrosion resistance, trivalent chromate-conversion film excellent, in particular, in corrosion resistance after heating.
Another object of the present invention is to provide a processing solution used for forming such a hexavalent chromium free, corrosion resistance, trivalent chromate-conversion film and a method for forming the film.
Moreover, it is also an object of the present invention to provide a method for forming such a film in which the same devices and processes used in the formation of the conventional hexavalent chromate film can be used as such without any modification.
The present invention has been developed on the basis of such a finding that the foregoing problems associated with the conventional techniques can effectively be solved by forming a trivalent chromate conversion film containing a hardly soluble cobalt oxalate salt and Si, while using a trivalent chromate-conversion processing solution (plating bath) having a specific composition, after depositing a zinc plating layer on a substrate.
According to an aspect of the present invention, there is provided a processing solution for forming a hexavalent chromium free, corrosion resistance trivalent chromate film on zinc or zinc alloy plating layers and the processing solution comprises:
a silicon compound;
trivalent chromium and oxalic acid in a mole ratio ranging from 0.5/1 to 1.5/1, wherein the trivalent chromium is present in the form of a water-soluble complex with oxalic acid; and
cobalt ions, which are stably present in the processing solution without causing any precipitation by forming a hardly soluble metal salt with oxalic acid;
wherein the solution reacts with zinc when bringing it into contact with the zinc or zinc alloy plating to form a hexavalent chromium free, corrosion resistance, trivalent chromate film containing zinc, chromium, cobalt, oxalic acid and SiO2 on the plating.
According to another aspect of the present invention, there is provided the foregoing hexavalent chromium free, corrosion resistance, trivalent chromate conversion film containing zinc, chromium, cobalt, oxalic acid and SiO2 and formed on zinc or zinc alloy plating layers, wherein the SiO2 content thereof ranges from 1 to 10 mg/dm2, the mass ratio of chromium to (chromium+zinc) [Cr/(Cr+Zn)] is not less than 15/100, the mass ratio of cobalt to (chromium+cobalt) [Co/(Cr+Co)] ranges from 1/100 to 40/100 and the mass ratio of the oxalic acid to (chromium+oxalic acid) [oxalic acid/(Cr+oxalic acid)] ranges from 5/100 to 50/100.
According to a further aspect of the present invention, there is provided a method for forming a hexavalent chromium free, corrosion resistance, trivalent chromate conversion film, which comprises the step of bringing zinc or zinc alloy plating into contact with the foregoing processing solution.